A method and apparatus for filtering seismic data are provided. More particularly, a method and apparatus are provided for attenuating surface wave generated noise in seismic-trace signals.
Among the more troublesome forms of noise or unwanted signal in seismic data are surface wave generated noise. Since surface waves travel on the earth's surface, they are not attenuated as strongly as body waves which propagate through the earth's subterranean formations. Hence, surface wave generated noise tends to dominate body wave generated signals in the seismic data. Unlike other forms of noise or unwanted signal, surface waves produce coherent noise which cannot be eliminated by conventional common depth point stacking techniques.
Surface waves, including Rayleigh Waves and ground roll, do have certain identifying characteristics which others have employed individually to attenuate surface wave generated noise. Key among these characteristics are their low apparent velocity. By impressing a velocity filter on seismic-trace signals, unwanted surface wave generated noise can generally be removed since surface wave velocities can generally be distinguished from body wave velocities. Further, in large seismic recording arrays, recording array geometry can be adjusted, based on such velocity differences, such that surface wave generated noise will be recorded out of phase at certain locations in the recording array. Subsequent summation of the recorded surface wave generated noise will combine out as a result of being out of phase and hence can be attenuated.
A second characteristic of surface wave generated noise is that its predominate energy is generally contained in frequencies less than 10 Hz. Consequently, a straight forward band limiting frequency filter can attenuate much of the unwanted surface wave generated noise. Although both velocity filtering and band limiting filtering are generally successful, there are areas in which they do not work. In formations characterized by a high apparent velocity, for example, limestones, surface waves apparent velocities can be coincident with normal body wave apparent velocities. Additionally, such surface wave generated noise can contain a significant portion of its energy in frequencies above 10 Hz. As such, velocity filtering and frequency band filtering are not always effective.
A third characteristic of surface wave generated noise is its elliptical retrograde particle motion. To exploit this characteristic, three component seismic data have been acquired with unitary signal station, three component geophone arrays. Each unitary signal station, multicomponent geophone typically comprises one geophone which responds to vertical motion and two geophones, orthogonally spaced one from the other, which respond to horizontal motion. The geophones of such multicomponent geophone are typically arranged in XYZ orthogonal axes.
The elliptical retrograde particle motion of surface wave generated noise evidences itself in that seismic-trace signals acquired by a horizontal geophone of the multicomponent geophone array lead seismic-trace signals acquired by a vertical geophone of the multicomponent geophone array. Consequently, a cross correlation of the horizontal and vertical components of the seismic-trace signals can produce a peak at which surface wave generated noise events occur. However, this approach assumes pure surface wave generated noise events with no other concurrent event. As those skilled in the art would expect, this approach can give ambiguous results when other concurrent events are also present. Moreover, this approach merely provides a method for detecting surface wave generated noise, not a method to remove it.
Another technique which takes advantage of the elliptical retrograde particle motion obtains a measure of rectilinearity of particle motion within a given time window of the multicomponent seismic-trace signals. The degree of rectilinearity of particle motion is measured by a ratio of principal axes of an ellipsoid fit to the three components of the seismic data. Using this technique, compressional and shear body waves form very elongated ellipses--almost a straight line. Hence, a gain factor employing the measure of rectilinearly can be used to enhance linear motions and suppress elliptical motions. Variations of such techniques have been employed with limited success because this technique requires true amplitude relation among the three component seismic-trace signals. Moreover, this technique requires rectilinearity, a condition which is rarely met. For example, a transverse horizontal geophone of the single station, multicomponent geophone array responds to both transverse horizontal motion and radial horizontal motion. More recently, a surface wave generated noise filter has been developed employing a predictive deconvolution technique and the elliptical retrograde particle motion of surface wave generated noise.
Applicant has developed a novel method and apparatus for attenuating surface wave generated noise in seismic data which overcome certain limitations of existing surface wave generated noise filtering techniques.